Semiconductor spatial light modulators (SLMs) are suitable for digital imaging applications, including projectors, televisions, printers, and other technology. A DIGITAL MICROMIRROR DEVICE (DMD) is a type of SLM invented in 1987 at TEXAS INSTRUMENTS INCORPORATED of Dallas, Tex. The DMD is a monolithic semiconductor device based on micro-electromechanical systems (MEMS) technology. The DMD generally comprises an area array of bi-stable movable micromirrors forming picture elements (pixels) fabricated over an area array of corresponding addressing memory cells and associated addressing electrodes disposed under the micromirrors. The addressing electrodes are selectively energized by a control circuit with a voltage potential to create an electrostatic attraction force causing the respective micromirrors to tilt towards the respective address electrode. In some applications, the micromirror may be provided with a voltage potential as well. One embodiment of a DMD is disclosed in U.S. Pat. No. 7,011,015 assigned to the same assignee of the present disclosure, the teachings of which are incorporated herein by reference.
The fabrication of the above-described DMD superstructure typically uses a CMOS-like process with a completed SRAM memory circuit. Through the use of multiple photomask layers, the superstructure is formed with alternating layers of aluminum for the address electrodes, hinges, spring tips, mirror layers, and hardened photoresist for sacrificial layers that form air gaps.
The monolithic nature of the design and build of the DMD pixel technology is associated with quasi-planar structures interacting electrostatically with the tilting micromirrors. This presents a problem with the ability to shrink structures while attempting to maintain electrostatic entitlement. In the end, the design becomes more and more sensitive to electrostatic torque delivery originating from the edges of planar members and all the variations that this can create.
The electrostatic efficiency of a torsional spatial light modulator is limited by an elevated address electrode that is parallel to the micromirror when the micromirror is horizontal and not tilted, but which address electrode is angled with respect to the micromirror when tilted toward the address electrode. Providing a higher bias operation to increase torque generation on each address side of the micromirror can provide complications, such as field gradient induced migration of species in the headspace which ultimately can cause failure of the SLM. It can also create shorting where rounded features of raised binge together with a high field (and field gradient) can result in either catastrophic or transient current which can sputter metal from the binge or completely open up the base of the vias. The CMOS node capabilities to deliver additional bias are also problematic as the paths are shrunk.